1. Field of the Invention
This invention relates to a process for preparing unsaturated carboxylic acids from unsaturated aldehydes in the presence of a phosphorus-molybdenum-alkali metal type catalyst.
2. Description of the Prior Art
Various catalysts have been known as being suitable for the gas phase catalytic oxidation of unsaturated aldehydes.
These include, for example, processes for preparing acrylic acid from acrolein by using a catalyst consisting of P, Mo and As (Japanese Patent publication No. 19260/1963) or by using a catalyst consisting of Mo, V, W and silicon (Japanese Patent publication No. 12129/1969). Some of these catalysts have been used on a commercial scale.
Many processes have also been suggested for preparing methacrylic acid. These include, for example, Japanese Patent publication No. 6605/1972 (a Mo-Ni-Ti catalyst) and No. 10773/1973 (a catalyst containing Mo and Tl); U.S. Pat. No. 3,686,294 (a P-Mo-As catalyst) and No. 3795703 (a P, Mo and alkali metal type catalyst); and Belgian Patent No. 817100 (a P, Mo and Sb type catalyst). From the viewpoint of industrial suitability, however, these catalysts are quite insufficient regarding selectivity and lifetime. Consequently, there is still a need for improved catalysts for preparation of methacrolein and/or acrolein.